There conventionally have been proposed porous films, each obtained by incorporating fine inorganic powders in a polyolefin resin, melting the resulting mixture into a film, and stretching the film in a uniaxial or biaxial direction. Such porous films are used for various purposes as synthetic paper and are excellent, in particular, in offset printability due to various factors such as running stability on a printer, reproducibility of halftone dots, and adhesion of ink. Compared with natural paper derived from pulp, such synthetic paper has characteristics such as excellent printing sharpness and excellent water resistance so that it is suited for use in commercial printed matters such as outdoor posters, catalogues and maps.
The polyolefin resin serving as a raw material of such synthetic paper is however derived from fossil materials and stable supply of it is not assured in the future. When it is buried without burning after use, it will remain in the soil for a long period of time. Thus, it poses a problem of disposal.
Appearance of a resin film using so-called “Green Pla (R)”, plastic mainly making use of a biomass raw material derived from natural resources instead of conventional resins derived from fossil materials has been expected and many reports on it already have been made (Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, Patent Document 5, Patent Document 6, Patent Document 7, Patent Document 8, Patent Document 9, Patent Document 10, Patent Document 11, Patent Document 12, Patent Document 13, Patent Document 14, Patent Document 15 and Patent Document 16). A printing paper usable in sheetfed offset printing must have no picking, have water resistance, have size stability, have less paper habit, have adequate wettability with water, have good ink adhesion, drying property and ink set, have adequate resilience (stiffness), have running stability on a printer, and have good reproducibility of halftone dots. Thus, it must satisfy many property demands. Films satisfying the above-described quality demands have not yet appeared.
A resin film using a homopolymer of an oxycarboxylic acid (so-called polylactic acid or the like) described in the above patent documents is fragile owing to a high modulus of elasticity of the resin and therefore easily breaks when a tension is applied. Compared with natural paper or polyolefin-based synthetic paper which is adequately soft and has flexibility, the above-described film is inferior in running stability on a printer in sheetfed offset printing, which severely squeezes the film on its path (such as between rollers/blankets) or in web offset printing •gravure printing in which a high tension is applied to the film between rollers.
A resin film having excellent strength and modulus of elasticity (flexibility) comparable to those of a polyolefin resin is available from an aliphatic polyester resin containing an aliphatic diol and aliphatic dicarboxylic acid other than a homopolymer of an oxycarboxylic acid, but the printing paper (Patent Document 4) does not seem to have a sufficient opacity necessary for printing sharpness owing to a small content of fine inorganic powders; or on the contrary, the printing paper (Patent Document 6) having an excessively high content of fine inorganic powders and having too many voids formed therein is only described in Examples. None is well suited as a printing paper. The former one has poor appearance as paper and is inferior in printing sharpness, while the latter one does not function as synthetic paper usable in actual printing on a printer because lack of resilience leads to appearance of wrinkles during running or low substrate strength leads to appearance of picking (picking, deprival of ink during printing).
In addition, laminate paper obtained by laminating an aliphatic polyester resin on both sides of natural paper and excellent in printability is disclosed (Patent Document 17). Natural paper at the cross-section of this laminate paper absorbs water, which causes peeling of the thin resin film from the natural paper serving as a substrate or unevenness and this laminate paper does not have sufficient water resistance suited for use in outside posters or the like. In addition, unevenness of the fibers of natural paper constituting the center layer appears as surface roughness so that it is inferior in halftone dot reproducibility during printing compared with synthetic paper, which makes the print image blurry. This paper is therefore not suited for high definition printing.    Patent Document No. 1: JP-A-05-209073    Patent Document No. 2: JP-A-09-031228    Patent Document No. 3: JP-A-09-208817    Patent Document No. 4: JP-A-09-272789    Patent Document No. 5: JP-A-09-291163    Patent Document No. 6: JP-A-09-291164    Patent Document No. 7: JP-A-09-291165    Patent Document No. 8: JP-A-10-119227    Patent Document No. 9: JP-A-10-202690    Patent Document No. 10: JP-A-11-105224    Patent Document No. 11: JP-A-11-129426    Patent Document No. 12: JP-A-2000-136299    Patent Document No. 13: JP-A-2001-049003    Patent Document No. 14: JP-A-2001-049004    Patent Document No. 15: JP-A-2002-194195    Patent Document No. 16: JP-A-2003-342404    Patent Document No. 17: JP-A-2003-220680